This application claims priority from commonly-assigned Republic of Korea Application Serial No. P97-17209 (filed May 6, 1997) and Serial No. P97-17211 (filed May 6, 1997).
The present invention relates to a technique for depositing a platinum film, which is used as a bottom electrode for memory cells or film sensors, and to a method of manufacturing such a film, particularly to a platinum film, which is deposited under an atmosphere containing an oxygen component (O2, O3, O2+N2, N2O, or mixtures thereof) together with an inert gas (Ar, Ne, Kr or Xe) so that its orientation and microstructure can be controlled, and to an electronic device comprising such a platinum film as an electrode and a method of manufacturing the electronic device. In the below description, oxygen component means O2, O3, O2+N2, N2O, or mixtures thereof. More particularly, the present invention relates to a technique improving characteristics of a platinum film in the course of manufacturing the platinum film used as a bottom electrode, wherein a preferred orientation of the platinum film is controlled toward a predetermined direction(s) and the platinum film is controlled to have a microstructure without any defects such as hillocks, pinholes and pores to be sufficiently useful as an electrode for memory cells, or ferroelectric sensor devices.
Semiconducting, dielectric, ferroelectric, superconductive and magnetic ceramic materials, used in electronic devices tend to become thinner in line with the trends of miniaturization, high-density integration and functional elevation of electronic ceramic parts or devices. Therefore, thin film type ceramic parts have been widely used in electronic industries. The substrates used for thin film type ceramic parts can be classified into three types. The first type comprises single crystal silicon generally referred to as silicon wafers. The second type comprises the other single crystals such as MgO, SrTiO3 or Al2O3. The third type comprises polycrystal materials such as alumina or diamond. Among them, silicon wafers have been widely used in the conventional manufacturing processes of various types of electronic devices such as memory devices, or sensor devices.
Polysilicon has been widely used as a bottom electrode material in the conventional memory cells without any critical problems. However, it is generally accepted that polysilicon cannot be used any more as a bottom electrode to manufacture DRAMs (Dynamic Random Access Memories) with over 1 Gigabit and FRAMs (Ferroelectric Random Access Memories) which is a new type of non-volatile memory, because high dielectric or ferroelectric oxide thin films such as perovskite structure oxides, bismuth-layered perovskite structure oxides, tungsten-bronze type structure oxide, ReMnO3 (Re: rare-earth element), and BaMF4 (M: Mn, Co, Ni, Mg, Zn), are used on FRAMs, on DRAM devices requiring high degree of integration over 1 Gigabit, or around the core part of all types of oxide thin film sensors or actuators. That is, forming such high-dielectric oxide films requires an oxidation atmosphere and high temperature (higher than 500xc2x0 C.), which may cause problems relating to the polysilicon. For example, if polysilicon is employed as the bottom electrode in a DRAM cell using high-dielectric material for a capacitor, serious problems may occur due to oxidation of the polysilicon under the high temperature (over 500xc2x0 C.) and oxidation atmosphere during formation of the high-dielectric oxide thin films. For this reason, platinum is being investigated for use in place of polysilicon as an electrode of a memory cell employing a high-dielectric or ferroelectric oxide, because platinum is stable under high temperature and an oxidation atmosphere.
However, depositing platinum thin films by means of conventional methods has been known to pose a number of problems. First, the interface between an insulating oxide layer and the platinum layer does not allow chemical bonding, thereby weakening the adhesion strength between the platinum film and the substrate. One of the attempts that has been made to solve this problem is using an adhesion layer between a platinum layer and an insulating oxide layer. A thin film composed of any one or two of Ta, Ti, TiN or W has been formed on an insulating oxide layer before depositing a platinum thin film so that the thin film composed of any one or two of Ta, Ti, TiN or W serves as an adhesion layer between the insulating oxide layer and the platinum film.
However, employment of this method is known to not only complicate the process of forming a bottom electrode but also generate additional problems. In particular, oxygen gas introduced during the post-annealing or high-dielectric/ferroelectric oxide film depositing process can diffuse through voids formed between grain boundaries in the platinum film. Because the grains of the platinum films deposited by conventional processes have vertical columnar structures with inter-columnar voids, oxygen introduced from the above-mentioned process can easily diffuse through the platinum film to the adhesion layer. The oxygen gas diffused through the platinum film then oxidizes the adhesion layer and forms an oxidized insulation layer such as TiO2 and Ta2O5 between the substrate and the platinum film. Consequently, the function of the platinum film as an electrode can become deteriorated or even lost. In particular, if the adhesion layer is formed from TiN, N2 gas is produced while an oxidized layer of TiO2 is formed on the surface of TiN layer, and the N2 gas can cause the platinum film to expand and become released from adhesion layer. This phenomenon is known as xe2x80x9cbuckling.xe2x80x9d
In a conventional depositing method of a platinum thin film, hillocks, pinholes or pores are formed on the platinum film after annealing treatment or deposition of an oxide film. These hillocks, pinholes or buckling cause either shorting of the circuits or heterogeneity of a high dielectric or ferroelectric oxide layer.
Because of these problems, the use of oxide conductors such as IrO2, RuO2, LSCO, YBCO, etc., and platinum-oxide hybrid structures such as IrO2/Pt, RuO2/Pt, LSCO/Pt, YBCO/Pt, etc., as a bottom electrode have been investigated. However, when the former is used as the bottom electrode, the surface is not smooth enough and/or leakage currents increase. Furthermore, in the latter case, the manufacturing processes become complicated.
A solution for these disadvantages of conventional arts was addressed in two Korean patent applications (Serial Nos. 94-31618 filed Nov. 26, 1994 and 95-40450 filed Nov. 8, 1995) in the name of the present applicants. It is well known that oxygen may be contained in the depositing atmosphere when employing an insulating thin film on a substrate. The working effect of the two inventions addressed in said Korean applications filed in the name of the present applicants suggests a new technique. According to these inventions, a platinum thin film is deposited on an insulating oxide layer on a silicon wafer in two steps. The first step is to form a platinum thin film containing oxygen, as opposed to platinum, under an oxygen-containing atmosphere. The second step is to form a platinum layer on the Pt films formed at the first step under an inert gas atmosphere. The gases incorporated in the film during the first step are removed by annealing the film to a temperature higher than the decomposition temperatures of platinum oxides (PtO2: higher than 450xc2x0 C., PtO: higher than 550xc2x0 C.). Through this annealing process, the oxygen contained in the platinum film during the deposition thereof is removed and the remaining platinum film becomes stable. As a result, an adhesion layer is not required and thus problems related to the adhesion layer are solved.
According to the description in said Korean applications, xe2x80x9can oxygen containing gaseous atmospherexe2x80x9d means a mixture of inert gas (Ar, Kr, Xe, or Ne) with either oxygen or ozone gas mixture, or a gas mixture containing oxygen. xe2x80x9cA platinum thin film containing oxygenxe2x80x9d means that oxygen is contained in the platinum layer, partially forming a platinum oxide, and partially forming an amorphous phase in the platinum layer.
In addition to the aforementioned disadvantages in the conventional arts, there still remains another unresolved problem that crystallographic orientations of platinum thin films are uncontrollable. It is well known that the properties of anisotropic crystals depend on their crystallographic orientations. The crystallographic orientations of oxide films formed on bottom electrodes depend on the crystallographic orientations of the bottom electrodes. Therefore, it is believed that controlling the preferred orientation of bottom electrodes is very important in controlling the preferred orientation of oxide films in order to have films with desirable physical properties.
It is expected in the pertinent technical field that, if a platinum thin film employed as a bottom electrode is deposited with a preferred (200) orientation, a ferroelectric oxide thin film, which is formed on the platinum thin film, could be mostly oriented to one direction (for example, c-axis). On the account of this preferred orientation, the working efficiency of the electronic device is expected to significantly increase, and its fatigue effect is expected to decrease.
It is known that a platinum thin film, which is deposited on an insulating oxide layer by the conventional methods, generally has a preferred (111) orientation. This is due to the fact that the plane with the minimum surface energy in metals with face centered cubic (FCC) structure is (111) and, considering only the surface energy at the depositing, the film is most stable if oriented toward (111).
Conventional methods for forming preferred orientation-controlled platinum films that have been suggested have limitations. One of such conventional methods is forming the platinum film on a single crystal substrate of the materials such as MgO, NaCl, KBr, SrTiO3, Al2O3, and LaAlO3. However, such a method not only is complicated in its process and incurs high unit costs for single-crystalline substrates but also is incompatible with the state-of-the art in manufacturing silicon-integrated circuits. Other conventional methods have formed orientation-controlled platinum films by depositing platinum on a glass substrate not on a silicon wafer, or by using a specially designed sputtering equipment which has an xe2x80x9cauxiliary electrodexe2x80x9d in order to deposit platinum film on silicon substrates. However, it has been reported that orientation-controlled platinum films deposited on glassy substrates have high resistivity (18 to 30 xcexcxcexa9-cm) since oxygen, which was introduced during the deposition thereof, has remained within the platinum films even after annealing for 10 days. Therefore, it has been thought to be difficult and impractical to apply this process to real manufacturing practices, due to the annealing condition.
It is seen from the above that the alternative and improved methods for forming preferred orientation-controlled platinum films onto silicon wafers as well as other substrates are needed. It is desirable that such methods be compatible with silicon-integrated circuit technology in some applications. It is also desirable that such preferred orientation-controlled platinum films have minimized pinholes, pores or hillocks in order to provide improved device performance.
The aforementioned drawbacks have been resolved by the invention disclosed in Korean Patent Application No. 96-7663 filed on Mar. 21, 1996 (corresponding U.S. application Ser. No. 08/688,521, filed Jul. 30, 1996) by the present applicants"" technology of depositing a platinum film having a preferred (200) orientation on a silicon wafer under an atmosphere containing oxygen. According to that invention, platinum thin films deposited on a silicon wafer with an insulating oxide layer have good adhesion strength and a preferred (200) orientation.
Therefore, to date, there has been no report of desirable results with depositing a platinum film on a silicon substrate in such a manner that the platinum film has a microstructure that can be controlled; an orientation that can be controlled to (111), (200), or (220); and an excellent conductivity desirable for application to DRAM, FRAM or sensor devices.
It is therefore an objective of the present invention to provide a method of manufacturing a platinum film, the orientation and microstructure of which can be controlled irrespective of the employment of an adhesion layer formed thereon, and such platinum film manufactured thereby.
It is still another objective of the present invention to provide a method of manufacturing an electronic device comprising an orientation-controlled platinum film and an electronic device manufactured thereby.
According to one aspect of the present invention to achieve the above objectives, the present invention provides a method of forming a platinum film on a substrate. The method includes the steps of: providing a substrate; depositing platinum on an upper surface of the substrate under an inert gas atmosphere containing an oxygen component (O2, O3, N2+O2, N2O, or mixtures thereof) to deposit a platinum film containing oxygen; and annealing the platinum film containing the oxygen component at a temperature between 450xc2x0 C. to 1,000xc2x0 C. which is higher than the decomposition temperatures of the platinum oxides to remove the gases incorporated into the platinum films during the deposition. Orientation of such a platinum film can be controlled by changing at least one of the following parameters: the partial pressure ratio of the oxygen component to the entire gas containing the inert gas and oxygen component, the temperature of the substrate during the deposition, and the annealing conditions. The platinum film can be formed through two or more deposition steps, according to specific embodiments.
In accordance with the present invention, the platinum film may be deposited by employing any one of the following methods: direct current or radio frequency (DC/RF) magnetron sputtering, DC/RF sputtering, metal organic chemical vapor deposition, partially ionized beam deposition, vacuum evaporation, laser ablation, and electroplating.
In other embodiments, the method forming a platinum film described above can be applied to a process for manufacturing an electronic device by forming a high-dielectric or ferroelectric oxide film on the platinum film. In this case, for example, the platinum film functions as a bottom electrode. Depending on types of electronic devices to which the film is applied, a functional intermediate film (such as an insulation layer, a conductive plug, an adhesion layer, or a diffusion barrier layer) may be provided between the platinum film and the substrate, according to various specific embodiments. In accordance with other specific embodiments, the present invention provides an electronic device with preferable characteristics required for memory cells or sensor devices.
The present invention along with its features and advantages will now be explained in detail with reference to the attached drawings illustrating preferred embodiments of the present invention.